This invention relates to radiopaque colorless glass ceramics, to compositions comprising such glasses and a polymerizable synthetic resin, and in particular to such compositions useful for dental fillings. To replace the conventional amalgam or gold dental fillings, compositions have been developed based on rapidly polymerizable synthetic resins, e.g., acrylate or methacrylate, and a pulverulent inorganic material, e.g., glass, embedded in the synthetic resin.
Heretofore, two groups of inorganic fillers have been added to the synthetic resins: (a) glasses distinguished by relatively high contents of BaO (cf. the article by Bowen and Cleek in "Journal of Dental Research," 1972, pp. 177-182); and (b) Li-Al silicates (DOS [German Unexamined Laid-Open Application] 1,570,971). The reason for the use of BaO-containing glasses is that barium exhibits a high absorption of short wavelength X-ray radiation, as utilized in dental diagnostics. As a result thereof, the composite synthetic resin-Ba-glass element has an absorption which clearly distinguishes same from the surrounding, natural tooth material or even from possible cavities.
The second group of fillers, namely the Li-Al silicates, do not possess this advantageous, high X-ray absorption, but, instead are distinguished by very low, frequently even negative, coefficients of thermal expansion. This property is very desirable insofar as low coefficients of thermal expansion of the filler material can compensate for the very high coefficients of expansion of the synthetic resins, so that the average coefficient of thermal expansion of the synthetic resin-Li-Al-silicate composite can easily be adapted to that of natural tooth substance. This is a very important advantage, since otherwise there is the danger that exposure to sudden temperature changes would result in mechanical stress within the tooth or mechanical stress which might even cause an unloosening of the filling from the tooth. (To avoid this problem, ground quartz glass, as well as Li-Al silicates, has also been utilized as the filling material.)
Whereas it would be desirable to provide a glass having both the desired high X-ray absorption and the described low coefficient of expansion, heretofore, no materials have been known which would sufficiently satisfy this combination of requirements. In glasses, a low thermal expansion can normally be attained only with very high contents of SiO.sub.2 which, in turn, cannot be coupled with the requirement of a high X-ray absorption. The number of other solid substances of very low thermal expansion is relatively limited. Among these substances, insofar as they are sufficiently resistant to chemicals, there is not a single one with a high X-ray absorption.